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There’s a lot to consider when writing an underwater adventure.  It gets even more difficult if you’re writing in a fantasy world.  That or it’s easier depending on how much you want to mess with the laws of nature.  We’ll try to be more general here.  I’m also going to say that there is probably a lot more than 7 tips for this undertaking.  Let’s try to cover as much grou . . . water as we can.

1. RESEARCH!  Just read up on what it’s like to be underwater.  We always remember that one can’t breathe, but many forget other factors.  If the water is murky or the heroes are going deep then visibility is an issue.  Past a certain point, the water pressure would crush an unprotected body.  Temperatures can be freezing or even boiling if there are volcanic vents.  The effects on the body should be researched as well to get an idea on if the heroes can function down there or if they will be weakened upon their return.
2. Preparation is key for the heroes.  This is more complicated than most other terrains because of all of the challenges.  There should be a build up to the adventure where the heroes are making plans and getting supplies.  Even if the author is a pantser, the characters shouldn’t be diving into the depths without a plan.  For one thing, they need to have a way to breathe and readers might not be willing to believe that they just happen to have the gear for doing so on-hand.  So, take even part of a chapter for the heroes to discuss the challenges.
3. Unless a hero is diving into the shallows, they aren’t going to run into any buildings within minutes.  Shipwrecks and sunken structures don’t normally hover in the water or stay near the surface.  If they did then more people would visit and they wouldn’t be mysterious.  An adventure that requires going underwater typically means searching a rarely, or never, visited area.  So, there needs to be time given towards the journey down, which helps with tension as well.
4. I mentioned that there’s going to be a visibility problem in murky or deep bodies of water.  Every location doesn’t have crystal clear water and even that has a depth limit before light can’t go any further.  This means that you need to consider darkness when designing the setting.  You can go for full darkness until a lit place is reached or having there be some light source.  Bioluminescence . . . How did I write that correctly on the first try . . . Anyway, this is when a living creature gives off light.  Many authors use this to create a brighter scene and explain how the characters with no natural ability to see underwater can notice things.  There may still be shadows and patches of darkness that creatures can emerge from, but this helps create a more mystical and alien scene instead of going for the tension of isolation.
5. Think about the creatures you want the heroes to encounter if they’re going to run into anything at all.  Will they be friendly or aggressive?  The reason for this is because these animals will always have the advantage.  They are naturally designed to function underwater, which includes quick and precise movements.  Adventurers are visiting and will either be in a machine or using magic to survive.  This does not mean they will be able to fight and move with the same level of skill as they would on land.  Aside from moving against the water, they have to change their perspective to a 360 view.  These animals can attack from any side, but also from above, below, and at other angles that are impossible on land.  So, think about your creatures because this will give you an idea as to how your heroes need to move.
6. Something we haven’t brought up yet deals more with the ocean than more contained bodies of water like lakes.  Though, rivers can have this issue as well even they don’t have the depth. I’m talking about CURRENTS!  Water typically isn’t standing still, so whatever is inside will be pushed along.  The only way to not get dragged away is to have the strength to fight against the currents.  This can be tiring for anyone who is too weak or just strong enough to fight.  Another method is to ride the currents and move along them, which takes knowledge that average adventurers wouldn’t have.  Also, this means they might not come up at the same location that they went down.  Same goes for whatever they are searching for since an object might not fall straight to the bottom and sink more at an angle.
7. As we’ll get into on Friday, you need to figure out how the characters will get underwater and stay there.  Anyone can swim down a bit and come back up for air, but these are adventures where the heroes need to stay down there.  So, are you going for a magical, technological, or combo solution?  Each one has benefits and weaknesses, so you need to think about where you want to go.  Also, the heroes need access to these options.  For example, if they can’t get a submarine then they have to take the magic route.  Doing otherwise would twist reality a bit too much for the reader.